There exist many applications in which it is advantageous to mark articles with identifying information that can be detected and/or read by electronic means. These applications include logistics (e.g. tracking of articles in storage and/or transport), sales (e.g. automated identification and billing/charging for items), and security (e.g. identification and/or authentication of documents or articles, including identity cards and negotiable instruments such as banknotes).
Barcodes are presently among the most widespread identification systems. The most common type of barcode includes a sequence of printed vertical bars and spaces to represent numbers and symbols. More recently, two-dimensional printed codes, such as QR codes, have become more widely employed, due to their ability to store greater quantities of information. Identification of such printed codes is a line-of-sight process using optical techniques, such as scanning the code with a laser reader apparatus or capturing and processing an image using a digital camera.
Due to their very low cost, and ease of fabrication (i.e. they may be printed using a wide range of conventional printing technologies) one- and two-dimensional printed codes are utilised in a very wide range of applications, including identification of products for retail sale, identification and checkout of library books, tracking of manufacturing and shipping movement, access and authentication (e.g. car park entry and exit), and so forth. However, optical technologies have a number of disadvantages for many applications of practical interest. For example, the requirement for a ‘line of sight’ between a printed code and the corresponding reader requires that a human operator generally be present to direct the reading process. Furthermore, it is generally not possible to detect the presence or content of printed codes unless a line of sight is available.
One technology which overcomes the abovementioned limitations of printed code systems is radio frequency identification (RFID). A conventional RFID ‘tag’, which is suitable to be fixed to articles requiring identification, includes a small antenna, and, optionally, a microchip. Such a tag can be ‘interrogated’ by a reader, which includes a transmitter and antenna for generating a radio frequency (RF) signal. The RFID tag is a transponder which receives the signal transmitted by the reader, and responds with a corresponding, detectable, RF signal of its own. This signal is received by the reader using either the same antenna employed for transmission, or a separate receiving antenna. The response generated by the RFID tag may include information, such as digital data, and may uniquely identify the tag, and therefore the article to which it has been affixed.
RFID tags including a microchip have the advantage that significant quantities of information can be stored, and transmitted back to the reader as a time domain signal. However, the cost of providing a silicon chip in every tag has limited the mass deployment of such RFID systems. A potential solution to this drawback is to omit the chip, to provide a chipless tag, and employ a fully printable conductive structure (e.g. using metallic inks), which can be interrogated using an RF signal to generate a ‘passive’ response.
Time domain chipless RFID tags are, however, limited in their ability to encode sufficient information within a small tag size. As a result, frequency domain chipless tags have been developed in which the tag comprises one or more resonant circuits that yield a unique frequency signature. While such systems have been demonstrated, with satisfactory results, under laboratory conditions, commercial deployment has proven to be more challenging. This may be, at least in part, due to the very limited data-encoding capacity of frequency domain chipless tags based upon printing using conductive ink on a paper substrate. The combination of a low Q factor of the tag's resonators, as well as a requirement for relatively accurate and reproducible printing, result in tags with a data-encoding capacity that is below industry requirements.
The use of electromagnetic (EM) imaging using mm-wave radiation has recently been proposed by the present inventors, as an alternative chipless RFID technology. The EM imaging technique differs from current time- and frequency-domain approaches in that no resonant frequencies are detected by the reader, such that the system shows greater immunity to low Q factors, and errors or inaccuracy in printing. The previously proposed technique for EM imaging employs strip-line polarisers on a substrate which backscatter or reflect mm-wave radiation in a substantially orthogonal polarisation state to the incident radiation. The polarisation dependence of the resulting received signal provides good immunity against ‘tag bending’ effect, and the presence of highly reflective items within the reading environment.
The abovementioned advantageous attributes of EM imaging techniques suggest significant potential for industrial application through a low-cost printable tag structure. However, the previously proposed technique has at least one notable limitation. Specifically, in order to obtain sufficiently high-resolution imaging for decoding of information using mm-wave radiation, the reader must be moved relative to the tag during interrogation, in order to create a sufficiently large aperture for synthetic aperture radar (SAR) signal processing. This requirement limits the range of applications in which mm-wave EM imaging RFID systems may be deployed in practice, and increases the required reading time, which may be problematic in high-volume scanning applications.
Furthermore, it is clearly desirable in any EM imaging RFID system to minimise the required tag size for a given volume of stored information and/or backscattered signal strength.
The present invention therefore seeks to address at least these issues relating to reader movement and tag size.
It should be noted that the foregoing information is provided to provide a context for the present invention, and to assist in understanding its features and benefits. However, this should not be taken as an admission that any of this background information forms a part of the prior art or the common knowledge of persons having ordinary skill in the field of RFID technology.